株选与穗选是玉米品种选育的基础环节。从1950s年代到2010s年代，我国玉米品种更替过程中，植株和穗部形态特征所发生的改变对玉米产量提高有着重要贡献。在过去的七个年代，从1950s年代到2000s年代穗高和穗位高呈小幅上升趋势，2010s年代出现下降；而穗位系数和穗下节间长呈明显下降趋势。玉米叶片变得更加直立，尤其是果穗上部的叶片，单株叶面积显著增加。1950s年代至2000s年代，叶向值增加到2010s年代出现下降趋势；雄穗的大小和抽雄与吐丝间隔天数都显著减少。相同栽培条件下，果穗直径、粒数和粒重从1950s年代到2000s年代呈上升趋势，2010s年代出现下降。我们发现，现代玉米品种具有较低的株高、穗位高和穗位系数增强了玉米的抗倒伏能力；具有直立的叶片特征提高了玉米的耐密性；具有较小的果穗及籽粒有利于灌浆后期快速脱水。建议把这些形态特征的选择标准作为未来适宜于机械化作业玉米品种选育的重点参考依据。

To cope with a highly heterogeneous light environment, photosynthesis in plants can be regulated systemically.  Currently, the majority of studies are carried out with various plants during the vegetative growth period.  As the reproductive sink improves photosynthesis, we wondered how photosynthesis is systemically regulated at the reproductive stage under a vertically heterogeneous light environment in the field.  Therefore, changes of light intensity within canopy, chlorophyll content, gas exchange, and chlorophyll a fluorescence transient were carefully investigated at the graining stage of maize under various planting densities.  In this study, a high planting density of maize drastically reduced the light intensities in the lower canopy, and increased the difference in vertical light distribution within the canopy.  With the increase of vertical heterogeneity, chlorophyll content, light-saturated photosynthetic rate and the quantum yield of electron transport in the ear leaf (EL) and the fourth leaf below the ear (FLBE) were decreased gradually, and the ranges of declines in these parameters were larger at FLBE than those at EL.  Leaves in the lower canopy were shaded artificially to further test these results.  Partial shading (PS) resulted in a vertically heterogeneous light environment and enhanced the differences in photosynthetic characteristics between EL and FLBE.  Removing the tassel and top leaves (RTL) not only improved the vertical light distribution within the canopy, but also reduced the differences in photosynthetic characteristics between the two leaves.  Taken together, these results demonstrated that maize plants could enhance the vertical heterogeneity of their photosynthetic function to adapt to their light environment; slight changes of the photosynthetic function in EL at the graining stage under a vertically heterogeneous light environment indicated that the systemic regulation of photosynthesis is weak at the graining stage.

玉米收获时籽粒含水率高是中国东北高纬度地区玉米生产面临的重要问题，这与品种熟期、区域气候条件以及栽培管理技术密切相关。延迟至冬季收获不能有效降低籽粒含水率以解决上述问题。2016至2017年，在黑龙江省大庆市试验点，连续观测了不同成熟型玉米品种生理成熟后籽粒田间干燥情况。采用两段线性模型对籽粒含水率与外界气象因子进行了阶段性分析。1)两段线性模型可以将各品种的籽粒干燥过程划分为两个不同斜率的单独线性干燥过程，且拟合精度良好。2)快速干燥阶段，温度越高，干燥速度越快。而大气水汽压条件对慢速干燥过程的速率有影响。3)干燥速率由快速干燥阶段转为慢速干燥阶段时的籽粒含水率以及气象因子在品种和年份之间不一致，这两者并非是干燥速率明显变化的关键因素。但霜冻后，气温<0℃会显著降低籽粒干燥速率。4)早熟品种生育期短，干燥时间得以延长，籽粒含水率显著低于中晚熟品种。由于气温下降迅速，籽粒的干燥速率显著降低，中晚熟品种难以在田间干燥至较低的含水率水平。因此，更换早熟品种并实施相应的栽培技术是解决高含水率问题的可行途径。

玉米机械粒收破碎率高是影响粒收质量的重要因素，本试验利用研磨法测试玉米子粒破碎对水分的敏感性，探寻玉米子粒破碎率最低的含水率，并对品种耐破碎性进行评价。在北京和新乡两个试点分不同播期种植17个玉米品种，系统测试子粒水分动态变化，并利用研磨法同步进行子粒破碎率测试，分析破碎率与含水率的相关关系。北京试点和新乡试点及两地总体样本子粒含水率 (x) 与破碎率 (y) 关系均符合二次曲线 (y=ax²+bx+c) 关系，其中，两地512个样本拟合方程为y=0.0796x²-3.3929x+78.779(R²=0.2646，n=512) ，由方程拟合可见，最低破碎值为42.62%，对应的子粒含水率为21.31%；设置90%的置信区间，子粒破碎率最低的含水率范围为19.7%-22.3%，与田间机械粒收最低破碎率出现的含水率值一致。以破碎率值最低点为界发现，在低含水率条件下，破碎率与含水率呈显著线性负相关；在高含水率条件下，破碎率与含水率呈显著线性正相关；由拟合曲线(y=ax+b)斜率和相关度可见，在子粒高含水率条件下，子粒破碎对水分的敏感性更强，相关度更高。利用各品种子粒破碎率与含水率二次曲线的积分值评价不同品种子粒破碎敏感性评价方法，在北京试点筛选出耐破碎性强的品种为郑单958和丰垦139，易破碎品种包括联创825、吉单66、利单295和京农科728；在新乡试点筛选出耐破碎品种为禾田1号、郑单958和丰垦139，易破碎品种包括泽玉8911、迪卡653和京农科728。两地共用的6个品种分类结果基本一致。以上结果表明，研磨法是一种稳定性较高的检测方法，可以用于品种破碎对水分敏感性和耐破碎性评价，为耐破碎玉米品种的选育与筛选提供支持。

玉米田间籽粒收获相对于穗收能够节省后续运输、晾晒和脱粒等环节的人工成本，然而粒收后籽粒含水率出现升高的现象，降低了籽粒品质。为明确收获前后籽粒含水率差异的原因，本研究利用黄淮海平原多年多点玉米粒收试验以及在籽粒不同含水率阶段的分期收获试验，观测收获前、后籽粒含水率，破碎率，杂质率以及植株各器官含水率。在多年多点试验中，411组测试样本表明，粒收作业后籽粒含水率较收获前含水率值平均高出2.2%。分期收获试验结果表明，当收前籽粒含水率低于23.9%时，收获前、后测试结果没有显著差异，而当收前含水率高于23.9%后，收获后籽粒含水率测试值显著升高；收获后籽粒含水率增加值与收前籽粒含水率、破碎率、杂质率呈极显著正相关。通常，黄淮海夏玉米区收获期植株成熟度低、籽粒含水率高，造成较多的破碎和杂质，进而导致收获后籽粒含水率测试值升高。因此，我们建议选择生育后期植株落黄快的品种，并适当延迟收获期，降低收前籽粒含水率，从而降低破碎率和杂质率，提高收获后籽粒品质，推动中国玉米粒收发展。

The uneven distribution of solar radiation is one of the main reasons for the variations in the yield gap between different regions in China and other countries of the world. In this study, different solar radiation levels were created by shading and the yield gaps induced by those levels were analyzed by measuring the aboveground and underground growth of maize. The experiments were conducted in Qitai, Xinjiang, China, in 2018 and 2019. The maize cultivars Xianyu 335 (XY335) and Zhengdan 958 (ZD958) were used with planting density of 12×10⁴ plants ha^–1 under either high solar radiation (HSR) or low solar radiation (LSR, 70% of HSR). The results showed that variation in the solar radiation resulted in a yield gap and different cultivars behaved differently. The yield gaps of XY335 and ZD958 were 8.9 and 5.8 t ha^–1 induced by the decreased total intercepted photosynthetically active radiation (TIPAR) of 323.1 and 403.9 MJ m^–2 from emergence to the maturity stage, respectively. The average yield of XY335 was higher than that of ZD958 under HSR, while the average yield of ZD958 was higher than that of XY335 under LSR. The light intercepted by the canopy and the photosynthetic rates both decreased with decreasing solar radiation. The aboveground dry matter decreased by 11.1% at silking and 21% at maturity, and the dry matter of vegetative organs and reproductive organs decreased by 9.8 and 20.9% at silking and by 12.1 and 25.5% at physiological maturity, respectively. Compared to the HSR, the root weights of XY335 and ZD958 decreased by 54.6 and 45.5%, respectively, in the 0–60 cm soil layer under LSR at silking stage. The aboveground and underground growth responses to different solar radiation levels explained the difference in yield gap. Selecting suitable cultivars can increase maize yield and reduce the yield gaps induced by variation of the solar radiation levels in different regions or under climate change.

Yield gap exists because the current attained actual grain yield cannot yet achieve the estimated yield potential. Chinese high yield maize belt has a wide span from east to west which results in different solar radiations between different regions and thus different grain yields. We used multi-site experimental data, surveyed farmer yield data, the highest recorded yield data in the literatures, and simulations with Hybrid-Maize Model to assess the yield gap and tried to reduce the yield gap by matching the solar radiation and plant density. The maize belt was divided into five regions from east to west according to distribution of accumulated solar radiation. The results showed that there were more than 5.8 Mg ha^–1 yield gaps between surveyed farmer yield and the yield potential in different regions of China from east to west, which just achieved less than 65% of the yield potential. By analyzing the multi-site density experimental data, we found that the accumulated solar radiation was significantly correlated to optimum plant density which is the density with the highest yield in the multi-site density experiment (y=0.09895x–32.49, P<0.01), according to which the optimum plant densities in different regions from east to west were calculated. It showed that the optimum plant density could be increased by 60.0, 55.2, 47.3, 84.8, and 59.6% compared to the actual density, the grain yield could be increased by 20.2, 18.3, 10.9, 18.1, and 15.3% through increasing plant density, which could reduce the yield gaps of 33.7, 23.0, 13.4, 17.3, and 10.4% in R (region)-1, R-2, R-3, R-4, and R-5, respectively. This study indicates that matching maize plant density and solar radiation is an effective approach to reduce yield gaps in different regions of China.

Yield gap analysis could provide management suggestions to increase crop yields, while the understandings of resource utilization efficiency could help judge the rationality of the management. Based on more than 110 published papers and data from Food and Agriculture Organization (FAO, www.fao.org/faostat) and the Global Yield Gap and Water Productivity Atlas (www.yieldgap.org), this study summarized the concept, quantitative method of yield gap, yield-limiting factors, and resource utilization efficiency of the three major food crops (wheat, maize and rice). Currently, global potential yields of wheat, maize and rice were 7.7, 10.4 and 8.5 t ha^–1, respectively. However, actual yields of wheat, maize and rice were just 4.1, 5.5 and 4.0 t ha^–1, respectively. Climate, nutrients, moisture, crop varieties, planting dates, and socioeconomic conditions are the most mentioned yield-limiting factors. In terms of resource utilization, nitrogen utilization, water utilization, and radiation utilization efficiencies are still not optimal, and this review has summarized the main improvement measures. The current research focuses on quantitative potential yield and yield gap, with a rough explanation of yield-limiting factors. Subsequent research should use remote sensing data to improve the accuracy of the regional scale and use machine learning to quantify the role of yield-limiting factors in yield gaps and the impact of change crop management on resource utilization efficiency, so as to propose reasonable and effective measures to close yield gaps.

目前缺水已成为制约作物产量进一步提高的主要因素，如何提高水资源利用效率(WUE)已成为我国农业亟待解决的问题。为研究玉米产量和水分利用效率对气象因素及群体变化的响应，我们在全国25个试验点安排了不同玉米群体的多点试验，不同的玉米群体由不同株型玉米品种(紧凑型和半紧凑型)和不同种植密度(30000株/ha、60000株/ha和90000株/ha)构成。研究发现：随着降雨量的增加，玉米产量呈现先增加后降低的趋势，水分利用效率随着降雨量的增加显著降低，60000株/ha种植密度的产量和WUE>90000株/ha>30000株/ha。通过分析WUE与主要气象因子的关系发现：水分利用效率与在各生育阶段(播种-吐丝、吐丝-成熟/收获、播种-成熟/收获和全年)的降水量(和R)呈显著负相关，与温度(T_m、T_M–m和GDD)、太阳辐射(Ra)呈正相关。为了研究不同玉米群体的水分蒸散特征，我们进一步安排了不同玉米群体的定点试验，研究发现：随着玉米群体的增大，玉米需水量显著增加，而土壤蒸发量显著减少。不同株型品种间及不同种植密度间玉米需水量和土壤蒸发量也存在显著差异。分析品种和种植密度对WUE的影响发现：选择抗旱杂交品种和适宜种植密度WUE分别提高21.70%和14.92%，表明选用抗旱杂交种比改变种植密度对WUE的提高作用更显著。综合考虑气象影响，采用ZD958等抗旱品种和60000株/ha的种植密度是提高我国北部玉米产量和WUE的有效途径。

本研究2017年和2018年的种植密度为12.0×10⁴ 株 ha^-1，在施氮量为0-450 kg ha^-1范围内设置4种不同氮肥处理；2019年种植密度分别为7.5×10⁴和12.0×10⁴ 株 ha^-1，在施氮量为0-765 kg ha^-1范围内设置18种不同氮肥处理。通过测定不同处理下玉米生育期、绿叶的叶面积指数（LAI）、籽粒含水量和籽粒脱水率指标，阐明施氮量对玉米籽粒含水量的影响。结果表明，施氮量从0增加到765 kg ha^-1，玉米吐丝期推迟约1天，成熟期推迟约1-2天。在生理成熟期和生理成熟期后，不同施氮量处理下籽粒含水量极差为1.9-4.0％。随着施氮量的增加，生理成熟后玉米籽粒的脱水率降低，但施氮量与籽粒脱水率之间没有统计学意义。生理成熟期叶面积指数与生理成熟后籽粒脱水速率之间无显著相关性。总之，施氮对玉米生理成熟期和成熟后籽粒含水量均有影响，但不同施氮量对籽粒含水量的影响较小。以上结果表明，在生产中不需要考虑施氮对玉米籽粒含水量的影响。

Increasing plant density is an effective way to enhance maize yield, but often increases lodging rate and severity, significantly elevating the risk and cost of maize production.  Therefore, lodging is a major factor restricting future increases in maize yield through high-density planting.  This paper reviewed previous research on the relationships between maize lodging rate and plant morphology, mechanical strength of stalks, anatomical and biochemical characteristics of stalks, root characteristics, damage from pests and diseases, environmental factors, and genomic characteristics.  The effects of planting density on these factors and explored possible ways to improve lodging resistance were also analyzed in this paper.  The results provide a basis for future research on increasing maize lodging resistance under high-density planting conditions and can be used to develop maize cultivation practices and lodging-resistant maize cultivars.

Competition is a common phenomenon in agriculture production.  Research on the relationship between competitive ability and crop yield is extensive, but the results have been inconsistent.  Few studies have focused on the relationship between population competitive intensity and yield of maize (Zea mays L.) cultivars.  The main objective of this study was to determine if a consistent relationship exists between maize yield and competitive ability.  A two-year field experiment was conducted, employing a de Wit replacement series design.  When two maize cultivars were grown in a mixture, yield was reduced for the modern cultivar and increased for the older cultivar.  In each replacement series, per plant level yield of each cultivar, and population level yield of the mixture, decreased with increasing proportion of the older cultivar.  Competitive ratio (CR) reflected differences in competitive ability of the three maize cultivars.  In each replacement series, population competition pressure (PCP) increased with increasing proportion of the older cultivar, indicating that the older cultivar was a strong competitor.  Biomass yield, grain yield, harvest index, thousand-kernel weight, and kernel number per plant, were negatively correlated with PCP.  Our results demonstrated that inter-cultivar competition affects maize productivity, and increasing PCP will decrease translocation of assimilates to grain and, ultimately, reduce yield.  Therefore, there is a negative correlation between population competitive intensity and yield performance in maize, breeders should develop a communal ideotype that would not perform well in competition in future. 

The calculation method of potential evapotranspiration (PET) was improved by adopting a more reliable PET estimate based on the Penman-Monteith equation into the standardized precipitation evapotranspiration index (SPEI) in this study (SPEIPM). This improvement increased the applicability of SPEI in North China Plain (NCP). The historic meteorological data during 1962–2011 were used to calculate SPEIPM. The detrended yields of maize from Hebei, Henan, Shandong, Beijing, and Tianjin provinces/cities of NCP were obtained by linear sliding average method. Then regression analysis was made to study the relationships between detrended yields and SPEI values. Different time scales were applied, and thus SPEIPM was mentioned as SPEIPMk-j (k=time scale, 1, 2, 3, 4,…, 24 mon; j=month, 1, 2, 3,..., 12), among which SPEIPM3-8 reflected the water condition from June to August, a period of heavy precipitation and vigorous growth of maize in NCP. SPEIPM3-8 was highly correlated with detrended yield in this region, which can effectively evaluate the effect of drought on maize yield. Additionally, this relationship becomes more significant in recent 20 yr. The regression model based on the SPEI series explained 64.8% of the variability of the annual detrended yield in Beijing, 45.2% in Henan, 58.6% in Shandong, and 54.6% in Hebei. Moreover, when SPEIPM3-8 is in the range of –0.6 to 1.1, –0.9 to 0.8 and –0.8 to 2.3, the detrended yield increases in Shandong, Henan and Beijing. The yield increasing range was during normal water condition in Shandong and Henan, where precipitation was abundant. It indicated that the field management matched well with local water condition and thus allowed stable and high yield. Maize yield increase in these two provinces in the future can be realized by further improving water use efficiency and enhancing the stress resistance as well as yield stability. In Hebei and Beijing, the precipitation is less and thus the normal water condition cannot meet the high yield target. Increasing of water input and improving water use efficiency are both strategies for future yield increase. As global climate change became stronger and yield demands increased, the relationship between drought and maize yield became much closer in NCP too. The research of drought monitoring method and strategies for yield increase should be enhanced in the future, so as to provide strong supports for food security and agricultural sustainable development in China. Received 12

In 2010, Chinese maize yields increased from 961.5 kg ha-1 in 1949 to 5 453.8 kg ha-1. This increase is the result of genetic improvements, an increase in nitrogen application, and refinement of planting densities. The objective of this study was to provide a theoretical basis for maize production research by analyzing the maize yield gain characteristics. Six varieties of maize were selected for the study; each selection is representative of a typical or commonly used maize variety from a specific decade, beginning from the 1950s and continuing through each decade into the 2000s. The selections and their corresponding decade were as follows: Baihe, 1950s; Jidan 101, 1960s; Zhongdan 2, 1970s; Yedan 13, 1980s; Zhengdan 958, 1990s; and Xianyu 335, 2000s. Each variety was planted under four different densities (37 500, 52 500, 67 500, and 82 500 plants ha-1) and four different nitrogen applications (0, 150, 225, and 300 kg ha-1) to study the effects on yield gain characteristics. The obtained results demonstrated that there was a maize yield increase of 123.19% between the 1950s variety and the 2000s variety. Modern Chinese maize varieties had a higher yield advantage. They also displayed the additional potential to acquire higher yield under increased planting densities and nitrogen applications. At the present cultivation levels (planting at 67 500 plants ha-1 with 225 kg ha-1 nitrogen application), the contribution types and corresponding yield increase percentages were as follows: genetic improvement, 45.37%; agronomic-management improvement, 30.94%; and genotype× agronomicmanagement interaction, 23.69%. At high-yielding cultivation levels (planting at 82 500 plants ha-1 with 300 kg ha-1 nitrogen application), the contribution types and corresponding yield increase percentages were as follows: genetic improvement, 31.30%; agronomic-management improvement, 36.23%; and genotype × agronomic-management interaction, 32.47%. The contribution of agronomic-management and genotype × agronomic-management interaction to yield increase would be larger with the corresponding management improvement. To further increase maize grain yield in China, researchers should further examine the effects of agronomic-management on maize yield and the adaptation of variety to agronomic-management.

Experiment was conducted at the Gongzhuling Experimental Station of Institute of Crop Sciences, Chinese Academy of Agricultural Sciences, Jilin Province, China, during 2009-2010. Six representative varieties of maize (Baihe in the 1950s, Jidan 101 in the 1960s, Zhongdan 2 in the 1970s, Yedan 13 in the 1980s, Zhengdan 958 in the 1990s, and Xianyu 335 in the 2000s) were each planted under two different densities (52 500 and 82 500 plants ha-1) and two different nitrogen application levels (150 and 300 kg ha-1). Root characteristics and distribution among soil layers were studied by the field root digging method. The results showed that root mass increased with the process of the growth and development of the plant, and it peaked at kernel filling stage, and decreased at maturity due to the root senesces. Root mass of different maize varieties from the 1950s to 1980s had a trend of increase, while it decreased for the modern varieties. Root length and root surface areas had the similar changing trend. The study suggested that early maize varieties may have root redundancy, and reducing root redundancy may be a direction for variety improvement for high yield. Root characteristics were affected by nitrogen application level and density; modern varieties were more suitable for higher fertilizer application level and density conditions. Root characteristics distribution among soil layers decreased by an exponent equation, but the regression coefficients of different varieties were different. Though the root length density (RLD) of every soil layer of different varieties also decreased by an exponent equation, there were large variations of RLD in every part of a layer.

To evaluate the effects of different conservation tillage modes on crop yield in various ecological regions, we interviewed peasant households in the Northeast China Plain, North China Plain, Chengdu Plain, and Northwest China Oasis, China,to analyze the influencing factors of the tillage methods on crop yield. An index set was determined from seven yield parameters based on expert consultation and relevant literature. A comprehensive evaluation of conservation tillage methods in the various ecological regions was carried out using fuzzy theory. The approaches with the highest scores were identified for each region: wide and narrow rotation planting with high stubble standing retention in the Northeast China Plain; seeding after rotary tilled of wheat under corn straw mulching and no-tillage seeding of corn and other crops under wheat straw mulching in the North China Plain; no tillage with high stubble retention in the Northwest China Oasis;and no-tillage seeding of wheat and other crops under rice straw mulching in Chengdu Plain. These research data provide a useful guide for the selection of conservation tillage methods for optimum yields in different regions of China.